PATTERNS OF DISCHARGE AND SUSPENDED SEDIMENT TRANSPORT IN THE WALNUT AND SQUAW CREEK WATERSHEDS, JASPER COUNTY, IOWA: WATER YEARS 1996-1998
Keith E. Schilling
Iowa Department of Natural Resources, Geological Survey Bureau
Technical Information Series 42, 2000, 47 p.
is the major pollutant affecting Iowa’s streams. Despite the magnitude of
sediment impacts on streams, little research has been conducted to determine the
nature of sediment transport in small Iowa watersheds. In the Walnut Creek and
Squaw Creek watersheds in Jasper County, Iowa, research as part of the Walnut
Creek Nonpoint Source Monitoring Project is focused on monitoring daily
discharge and suspended sediment in paired, agricultural watersheds.
The Walnut Creek project was established in 1995 as a nonpoint source
monitoring program in relation to watershed habitat restoration and agricultural
management changes implemented by the U.S. Fish and Wildlife Service (USFWS) at
the Neal Smith National Wildlife Refuge. The monitoring program utilizes a
paired-watershed approach, with Walnut Creek as the treatment watershed and
neighboring Squaw Creek as the control. Standard U.S. Geological Survey gaging
facilities are located at upstream and downstream locations on Walnut Creek and
downstream on Squaw Creek. The
purpose of this report is to present the results of three years of daily
discharge and suspended sediment monitoring in Walnut and Squaw Creek watersheds
and to examine the timing, frequency and magnitude of discharge and suspended
Daily discharge and suspended
sediment transport in both watersheds was very flashy, responding rapidly to
precipitation and snowmelt events. This
pattern is typical of incised channels where flood events are contained within
the channel and rapidly transport suspended sediment downstream. Five days in
any given year accounted for 60 to 80% of the annual sediment load in both
watersheds. Discharge and suspended sediment loads are highest in May and June,
which accounted for 40 to 50% of the annual discharge and approximately 60% of
the annual suspended sediment. The February to July period accounted for 98% of
the annual sediment total. Maximum
peak flows and sediment loads and a higher proportion of annual sediment loads
migrating during one- and five-day periods were observed in Squaw Creek than
Walnut Creek. Watershed morphology
and land-use differences, including prairie restoration in Walnut Creek, may
contribute to these differences.
A major source of
sediment is streambank erosion of Holocene alluvium and post-settlement
materials (up to 50% of the annual total). Streambed downcutting contributes
little to sediment loads due to base level control provided by resistant Pre-Illinoian
till. Other sources of sediment include contributions from upland sheet and rill
erosion and concentrated flow erosion in gullies and tributary channels. Eroded
sediment stored within the channel may take many years to exit the watershed.
A long-term monitoring record is needed to detect changes in the Walnut
Creek watershed resulting from the land restoration efforts.
The long timeframe is necessary for factoring out influences of climate,
variability in morphology and land use between Walnut Creek and Squaw Creek and
the effects of historical sediment storage.
The Walnut Creek project may be capable of detecting changes more quickly
than other projects due to the type and magnitude of land-use change being
implemented, the presence of fewer tiles in the watershed and the dual gages
located on the channel.
At a minimum, suspended sediment sampling should be conducted daily during the period February through July to characterize the flashy behavior observed in sediment loading during this time. In the August through January period, reduced sampling frequency, or estimating loads from daily discharge or turbidity can be used to estimate monthly sediment loads.